I need a clarification on these Actions.

Lets say a Beastman goes at the 2 success mark for initiative and gets a succesful attack.

Can a player try to parry or block in response even though the Beastman hasn't completed the maneuvre?

Hi.

You parry, block and/or dodge before any dice for the attack is rolled. You use these actions in the beastman turn, when he has declared you as a target, but before any dice are rolled for the attack.

Am I right in assuming that when the player's turn comes around, he only gets to do a maneuvre right?

No, the active defences are reaction cards that you play in the enemy turn (when you're declared the target of an attack). This does not prevent you from making an attack (or any other action) in your turn. But the enemy you're attacking could use their defence cards to block/dodge/parry the attack. You can even use more than one active defence against a single attack.